Impact of do‐it‐yourself air cleaner design on the reduction of simulated wildfire smoke in a controlled chamber environment

Abstract During wildfire smoke events public health agencies release advisories to stay indoors, close doors and windows, and operate a portable air cleaner (PAC). The do‐it‐yourself (DIY) air cleaner consisting of a box fan and a furnace filter is a widely used low‐cost alternative to commercial PACs because of its increased accessibility. In this study, we evaluate the clean air delivery rate (CADR) of different DIY air cleaner designs for reducing simulated wildfire smoke and identify operating parameters that may impact their performance and use. The simplest formulation of a DIY air cleaner (box fan with taped on minimum effectiveness reporting value – [MERV] 13 furnace filter) had a CADR of 111.2 ± 1.3 ft3/min (CFM). Increasing the fan flow by changing the fan type, increasing the fan setting, or reducing the pressure drop across the filtering surface increased the CADR. Large increases in CADR could be obtained by using a shroud (40%), using a 4″ thick filter (123%) using two filters in a wedge shape (137%), or using four filters in a Corsi‐Rosenthal (CR) box design (261%). The CADR was greatly reduced with filters heavily loaded with smoke, pointing to the need for frequent filter changes during smoke events.

availability during smoke events. 12 The recurrent need for cleaner air indoors during smoke events and limited accessibility for all impacted communities has led to the widespread use of do-it-yourself (DIY) air cleaners, also called filter fans. Numerous examples and guides exist online with the most common design consisting of a 20″ box fan with a single furnace filter attached to the fan inlet. Safety concerns on using a box fan with the filter in an aftermarket modification prompted a safety evaluation by Underwriters Laboratory.
They demonstrated that even under a worst-case scenario, both fan sides completely obstructed, resulted in no unsafe temperatures.
The low cost and evidence of safe operation may lead to more widespread DIY air cleaner use. 13 Some local air quality agencies have begun recommending that members of their communities make DIY air cleaners to improve indoor air quality during smoke events, with some organizations distributing DIY air cleaners 14 or providing the raw materials to facilitate their use during smoke events (e.g., Puget Sound Clean Air Agency).
DIY air cleaners have also risen in popularity as a low-cost method to reduce risk of COVID-19 transmission indoors, 15 with further design improvements to increase their cleaning capacity. These designs optimize the filtration surface area by using 4 or 5 filters attached to a single box fan (Corsi-Rosenthal Box, CR Box) increasing the filtered air flow. 16 Although there are multiple designs and guides for DIY air cleaners, there is very little information on their ability to deliver clean air and how their effectiveness may compare to more costly commercial PACs.
PAC effectiveness can be evaluated by measuring the single pass removal efficiency (fraction of particles removed from the inlet concentration) or by measuring the particle concentration decay due to the device. 17 Effectiveness is often reported as the clean air delivery rate (CADR) quantified in terms of clean air flow in ft 3 /min (CFM) and sometimes in m 3 /hr. Initial DIY air cleaner evaluations have focused on COVID risk reduction and have found them to be very effective at reducing particle concentrations. Reported CADRs for DIY air cleaners range from 49 CFM 18 to 825 CFM, 19 depending upon the fan/filter features and particle characteristics. CADR increased with increasing fan speed, [18][19][20] increasing filter thickness, 20 increasing filter rating (i.e., minimum effective reporting value -MERV), 20 increasing number of filters used 18,20 and increasing particle size. 21 However, there is large variability in the reported CADR for the basic design of a single MERV 13 filter with a box fan operated at high-speeds: 80, 18 300 20 and 330 CFM. 22 Similarly, a large range of CADRs (168-825 CFM) have been observed for the 5-filter CR box. [18][19][20][21] The range in reported CADRs may be impacted by specific design features used in each study (e.g., filter or fan manufacturer), but also by the evaluation method. For example, Pistochini and  The CADR for each design/condition was measured using the AHAM test protocol AC-1 23 with at least three replicates for each condition. We deviated from the test protocol, which is carried out in a sealed chamber with no external air flowing in, by supplying a continuous low flow to the chamber as makeup air for instrument samples. The natural PM 2.5 decay of the open chamber is larger than the closed room described in AC-1 23 and is accounted for by subtracting the natural PM 2.5 decay rate from the decay rate when the air cleaner is operating. Natural decay measurements were made periodically throughout the testing with the target PM 2.5 concentration to ensure representative values were used in the CADR calculation.
The basic DIY air cleaner was modeled after those provided by air quality agencies (i.e., the Puget Sound Clean Air Agency) to their communities or through popular online construction videos or blogs. We used one 20″ 3-speed box fan model A (83 W, 1820 CFM) for most experiments as it is easily obtained at most hardware stores. The impact of fan type was determined by using two other box fans, model B nominally 100 W and model C nominally 75 W.
These two fans are typical of commercially available models with flows of approximately 2000 CFM for the 100 W fan and 1800 CFM for the 75 W fan. Fan technical specifications are listed in Table S1.
All fans had three speed settings designated here as 'low', 'medium', and 'high' (fan label of '1', '2', and '3'), corresponding to increasing flow and increasing power draw. In most experiments 1″ thick MERV 13 furnace filters with electrostatic media (electret filters) from a single manufacturer were used. Additionally, 4" MERV 13 filters and 1" MERV 11 filters from the same manufacturer were used to evaluate the impact of filter type. The DIY designs that were evaluated are summarized in Table 1 and shown in Figure S2. Test Facility. 30 The facility was filled with smoke from small batches of pine straw and three air cleaners were operated simultaneously to reduce the smoke concentrations and load the filters. The process was repeated until the air cleaners ceased to reduce smoke concentrations in the chamber. The amount of smoke deposited was determined from a pre-and post-weigh of the filters on a balance (Scout, Ohaus). Additionally, the pressure drop across the smoke loaded filters was measured by a commercial laboratory (E-Spin Technologies).
A lower-cost, small commercial air cleaner with an AHAM CADR of 100 (CFM) for tobacco smoke was tested to compare the PM 2.5 removal effectiveness, sound, and cost of operation to the different DIY air cleaner designs. The statistical significance of comparisons between different air cleaners and designs was determined using a two sample T-test at a significance level of α = 0.05.
The material cost of the air cleaner was estimated from current prices on major online retailers (e.g., Amazon) and retailers with an online presence and physical locations in many rural areas (e.g., Ace Hardware). The median price of the limited survey done in April of 2022 was used to calculate the price per unit of each material needed to construct the air cleaner (Table S2). The operating cost was estimated by the power consumption for each air cleaner assuming a

| General observations
PM 2.5 concentrations were moderately repeatable within the chamber for both high and low concentration conditions, with approximately 30% variation in the PM 2.5 concentration across replicate tests. PM 2.5 concentrations were reduced to baseline levels between tests by the air cleaner, but the gas phase species (e.g., CO or THC) slowly increased in the chamber as multiple tests were run each day. The natural decay rate in the chamber was measured routinely throughout the study and was 0.012 ± 0.001 min −1 (n = 32).
The natural decay varied by 8.3% over the duration of the study due to changes in the instrument sample flow, as the number and type of instruments pulling samples from the chamber varied.

| Factors impacting DIY performance
Unlike commercial air cleaners, DIY air cleaners can be constructed and used in different ways that may impact CADR. We investigated a range of PM 2.5 concentrations, design features, and filter loading conditions to better understand what factors were most important in determining the CADR (Figure 1). The single MERV 13-1″ filter taped to a box fan was used as the baseline scenario to which all other conditions were compared. This baseline design is the simplest to construct, extensively publicized, and, in some cases, distributed by air quality agencies. It is therefore likely to be most widely used for reducing wildfire smoke. The baseline DIY design had CADR from 79.7-111.2, depending on fan speed (Table 2), which is comparable to a small capacity commercial air cleaner suitable for small rooms.
We also included the CR box in the evaluation as it has increased in popularity as a low-cost method to reduce COVID transmission risk indoors 31 and has been evaluated in several other studies. 18-21

| Initial PM 2.5 concentration
The CADR tended to increase with increasing initial PM 2.5 concentration, but only for designs with high CADR (Figure 2). The baseline scenario (MERV 13 1″ filter, attached by either tape or bungee cord) exhibited minimal variation in CADR across a wide range of initial   17 but this may reflect a shift to larger particle sizes at higher concentrations, which may be removed more efficiently with MERV 13 filters than the smaller particles at lower concentrations. The particle size was not measured but a proxy through a size ratio (PM 2.5 / PM 10 ) derived from PurpleAir sensor measurements suggests that particle size increases with PM 2.5 concentration ( Figure S3). However, the DustTrak reported nearly identical concentrations for PM 1 , PM 2.5 , PM 4 , and PM 10 , suggesting that all the particles were in the PM 1 size fraction. Although both instruments use light scattering to measure and predict size fractions, their design and calculation algorithm clearly differ, and additional sizing measurements would be needed to determine how the size may vary with concentration in the chamber.

| DIY air cleaner operation: Fan setting and fan placement
The CADR was strongly dependent on fan setting with the highest fan setting providing the greatest CADR but also the largest noise levels and highest power draw (

| DIY air cleaner design: Fan model, MERV rating, and filter configuration
The fan model had a small but significant impact on the CADR. The The MERV rating of the filter strongly impacted the CADR with a 43% reduction in CADR by switching from a MERV 13 to a MERV 11, with almost no change in the power draw or the device noise.
Although MERV 11 filters can sometimes be marketed as effective at removing smoke, the specifications for the MERV 11 rating require only 20% efficiency in removing particles smaller than 1 μm. 29 Wildfire smoke generally has a geometric mean diameter approximately 0.3 μm, 5 and most smoke particles fall into the range where MERV 11 is least effective, translating to a lower CADR.
A thicker filter (4″ compared to 1″) with the same MERV 13 rat-

| Filter condition
The filter condition had a strong impact on DIY air cleaner effective- CFM). Therefore, it is unlikely that the low CADR is due to the quantity of smoke deposited on the filter. It is likely that the low CADR for the smoke loaded filters is due to the electrostatic filters that were used in this study. Fresh smoke particles can carry charge 34  and commensurately had the highest CADR. Fans A and C had similar noise levels and power draws, but fan C had a 12% higher CADR.

| Comparison with other studies
The CADR measured in this study are compared to other studies evaluating DIY air cleaners in Figure 3. There is a consistent increase in CADR with increasing fan settings across studies ( Figure 3A), but the CADR for a single fan setting varies greatly, in part due to different design aspects. This can be seen when comparing CADR for the highest fan setting for different designs and studies in Figure 3B.
There is a clear trend of increasing CADR with increasing filter thickness and increasing number of filters used in the design, but large differences remain across studies examining the same configuration due to the CADR estimation method. The impact of test aerosol is seen by Zeng et al. 21 that reported increasing CADR for a CR box with increasing incense particle size (0.09-1, 0.5-3, 5-11 μm) compared to Dal Porto et al. 19 for the same CR box design with 0.5-5 μm

| Implications
This study has shown that DIY air cleaners can reduce smoke concentrations indoors and is a viable approach to improve indoor air quality during smoke events and provides CADR that are comparable to lower-cost commercial air cleaners sized for smaller rooms.
Designs that minimized the pressure drop across the fan by increasing the filter surface area (e.g., use of one 4″ thick filter, 2 or 4 filter designs) were able to achieve the highest CADR without any increase (or decrease) in noise levels or power consumption. These designs led to an increased CADR that were comparable to more commonly marketed commercial air cleaners (approx. 300 CFM) that is more appropriate for larger living spaces.
The CR box was particularly effective in reducing PM 2.5 concentrations with CADR nearly four times that of DIY air cleaner with a single MERV 13 1″ filter. In our small room-sized chamber, the PM 2.5 concentration was reduced from 80 μg/m 3 to below 10 μg/m 3 in approximately 6 min. However, this design is more complicated to construct and may take up a larger footprint, which may prevent widespread adoption of this design, despite the dramatically increased CADR from the baseline design of a single MERV 13 1″ filter.
The cost to purchase commercial air cleaners can be a major barrier in low-income communities and is one reason why DIY air cleaner designs have been so extensively publicized. Many people may already own a box fan and the air cleaner could be constructed with minimal cost by purchasing filters. Additionally, the lower material cost makes them attractive to air cleaner distribution programs run by local air quality agencies, community groups, or other nonprofit organizations. We compared the DIY air cleaner designs and a low-cost commercial air cleaner by the cost per unit of clean air delivered to identify the lowest cost and effective design ( Figure 4).
Furthermore, we categorized the costs into: (a) the initial cost to construct or purchase the air cleaner and (b) an estimated operating cost, which is the price of the electricity used to power the air cleaner. larger space that it takes up, and the greater effort involved in constructing the air cleaner compared to the commercial air cleaner.
Noise is a well-known major barrier to air cleaner use as is the draft they may produce, 37 but complexity of design is an additional consideration for DIY air cleaners since the user must also construct the cleaner. The CR box provides the best value (i.e., lowest initial and operating cost per CADR), but is more difficult to construct and takes up a larger amount of space. The single filter design using a MERV 13-4″ filter has a lower CADR but requires less space and is easier to construct. Despite a higher initial cost, the operating cost of the CR box may be lower since lower fan settings (and lower power draw) can effectively clean the same area as a single filter version.

| Limitations and suggestions for further study
A limitation of this study is the small number of fans and filter types investigated. The combinations tested here are a small fraction of the potential DIY air cleaners that can be constructed given the multiple types of box fans and numerous types of filters that are commercially available. This study focused on the simplest designs that may be most likely to be constructed and so combinatorial designs were not tested. Further studies could look at designs that combine varying number of filters, filter rating, filters without electrostatic media, and filters from other manufacturers to identify the most cost-effective design parameters. Although we showed that loaded filters caused a sharp decrease in the CADR, the heavy loading state used here is likely unrealistic for normal use and more study is needed to identify how the CADR changes over time as the filter load increases with more realistic smoke concentrations.
Testing in real environments is needed to truly measure the efficacy of DIY air cleaners, including the impact of noise levels and air flows on user behavior, which may greatly impact the achievable PM reductions. Moreover, the effectiveness of the DIY air cleaner in reducing symptoms is still needed to understand the benefits in addition to the costs associated with their routine use. Commercial air cleaners have been largely shown to be effective in reducing PM, but this has not always translated to improved health. 11 However, given the sometimes high indoor PM concentrations observed near wildfires, 38 DIY air cleaners may be very useful at improving indoor air quality conditions during smoke events.

| CON CLUS IONS
This study has shown that DIY air cleaners can be a cost-effective approach to reducing smoke concentrations. The CADR was minimally impacted by different approaches to attach the filter to the fan showing that DIY air cleaner performance may be largely independent from how the user constructs them. The most costeffective designs were those with multiple filters, but the use of a single 4" MERV13 filter was also highly effective and may be more suitable for small areas with minimal floor space that cannot accommodate the multi-filter designs. More study is needed to identify the optimal time to change filters and the optimal filter to use, particularly for wildfire smoke as the electrostatic filters used here were found to lose their filtering ability at much lower levels of PM loading compared to dust. This highlights the need for frequent filter changes during smoke events when they may quickly load up with PM.
F I G U R E 4 Initial cost of materials to build or purchase each air cleaner per unit CADR and estimated weekly operational cost assuming 8 h of use per day, 7 days a week and an electricity rate of 23.58 ¢/kWh. 39